Recently, due to a progress in micro processing techniques, it becomes possible to form a fine uneven structure of nano scale on a surface of a molded article. The fine uneven structure of nano scale exhibits a structure-derived effect such as anti-reflection effect referred to as a moth-eye effect or a water repellant effect referred to as a lotus effect, and thus industrial use of a fine uneven structure of nano scale is actively made.
There are various techniques for forming a fine uneven structure on a surface of a molded article. Among them, a method of transferring a fine uneven structure formed on a surface of a mold to a surface of a main body of a molded article is suitable for industrial production because a fine uneven structure can be given to a surface of a molded article with few simple steps. Recently, as a method for simple manufacture of a large-area mold having a fine uneven structure on a surface, a method for forming an oxidized coating film having a plurality of micropores (anode oxidized porous alumina) by anodic oxidation of an aluminum substrate has drawn attention (see, Patent Documents 1 and 2, for example). The oxidized coating film formed by anodic oxidation has an increasing interval (pitch) between micropores in proportion to an applied voltage. From the viewpoint that the interval between micropores can be relatively simply controlled, the aforementioned method is suitable as a method for manufacturing a mold.
However, when a mold is manufactured by using anodic oxidation, a method by which anodic oxidation is performed in two divided steps is suitable to have both micropore depth and regular arrangement that are preferred for the mold (hereinbelow, it is also described as a “two-step oxidation method” in this specification). Specifically, by sequentially performing the following step (1) to step (3), micropores preferred for a mold are obtained.
Step (1): The surface of an aluminum substrate is subjected to anodic oxidation to have a regular arrangement of micropores while ignoring the micropore depth.
Step (2): A portion or all of the oxidized coating film formed by the step (1) is removed.
Step (3): The aluminum substrate is subjected again to anodic oxidation after the step (2) to form micropores with a certain depth while maintaining the regular arrangement.
When a mold is manufactured according to the two-step oxidation method, the thickness of an oxidized coating film that is formed by the step (1) (hereinbelow, also described as an “initially formed oxidized coating film”) is preferably neither too thick nor too thin. In other words, when the oxidized coating film is thin, macro size irregularities like remnants of mechanical processing of an aluminum substrate remain even after the step (2) (for example, wrinkles generated by cutting process). When a mold with remnants of mechanical processing is used, the remnants of mechanical processing are also transferred on a surface of a main body of a molded article, and they become the reason of poor appearance. On the other hand, when the oxidized coating film is thick, the macro size irregularities like a step in a grain boundary of an aluminum substrate, which occurs after the step (2), become so significant that they can be visually recognizable. When a mold with a significant step in a grain boundary is used, the macro size irregularities like a step in a grain boundary are also transferred on a surface of a main body of a molded article, and they become the reason of poor appearance.
As described above, neither too thick nor too thin oxidized coating film formed in the step (1) is appropriate for use in a mold.
Although it may vary depending on a size of a grain of an aluminum substrate used for anodic oxidation or a method for mechanical polishing, when the initially formed oxidized coating film has a thickness in the range of 0.5 to 10 μm, there is generally no problem of using it as a mold. Thus, in order for an oxidized coating film to be formed within such range, it is necessary to suitably adjust an integrated quantity of electricity generated by anodic oxidation by controlling time or current density for anodic oxidation.
In Patent Document 1, for example, the thickness of an oxidized coating film is controlled by suitably modifying the time for anodic oxidation when an oxidized coating film in which an interval between micropores is 100 nm is formed by using an oxalic acid electrolytic solution having a concentration of 0.3 M and a temperature of 17° C. and performing anodic oxidation with an applied voltage of 40 V. However, under such conditions, the current density significantly increases and the rate of forming an oxidized coating film is significantly fast when the voltage is high. As such, it is difficult to control the oxidized coating film at 10 μm or less. The applicable applied voltage is less than 70 V at most.
In Patent Document 2, anodic oxidation is performed by using an oxalic acid electrolytic solution having a concentration of 0.05 mol/L and a temperature of 3° C. and an applied voltage of 80 V. By suppressing the concentration or temperature of an electrolytic solution, the current density is lowered to enable anodic oxidation at an applied voltage of 80 V. There is no description in Patent Document 2 relating to the thickness of an oxidized coating film. However, although the thickness is suitable, special equipment for maintaining the electrolytic solution at a low temperature of 3° C. is required in order to make the condition be industrially feasible, and thus it is not economically feasible.
As a method for controlling current density while maintaining the applied voltage, a method of modifying an electrolytic solution may be considered. In principle, by suitably selecting various electrolytic solutions that are described in Patent Documents 1 and 2, it is possible to easily control the thickness of an oxidized coating film without using special equipment for maintaining an electrolytic solution at a low temperature.
However, when an oxidized coating film is formed by using phosphoric acid or the like as an electrolytic solution, it is possible to increase the applied voltage to 80 V or higher, but problems like uneven thickness of an oxidized coating film or extremely disordered diameter of a micropore are yielded, and thus it is difficult to form an oxidized coating film that is suitable for use in a mold.